feature/required: add ValidateRequired method

This wraps the raw UnknownRequiredFeatures method and returns a proper
error type to enable more exact testing.

feature/required.go

@@ -0,0 +1,37 @@
+package feature
+
+import (
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+// ErrUnknownRequired signals that a feature vector requires certain features
+// that our node is unaware of or does not implement.
+type ErrUnknownRequired struct {
+	unknown []lnwire.FeatureBit
+}
+
+// NewErrUnknownRequired initializes an ErrUnknownRequired with the unknown
+// feature bits.
+func NewErrUnknownRequired(unknown []lnwire.FeatureBit) ErrUnknownRequired {
+	return ErrUnknownRequired{
+		unknown: unknown,
+	}
+}
+
+// Error returns a human-readable description of the error.
+func (e ErrUnknownRequired) Error() string {
+	return fmt.Sprintf("feature vector contains unknown required "+
+		"features: %v", e.unknown)
+}
+
+// ValidateRequired returns an error if the feature vector contains a non-zero
+// number of unknown, required feature bits.
+func ValidateRequired(fv *lnwire.FeatureVector) error {
+	unknown := fv.UnknownRequiredFeatures()
+	if len(unknown) > 0 {
+		return NewErrUnknownRequired(unknown)
+	}
+	return nil
+}